Boiler Failure Due to Oversizing of Boiler feed Pump

SAFETY VALVE SET PRESSURE & RELIEVING CAP.CALCULATIONS
PROJECT : M/S ABC Limited
INPUTS
Steam generation rate Nett = 35000 kg/h
Saturated steam flow for deaerator = 400 kg/h
Main steam pressure = 44 kg/cm2 g
Calculated Pressure drop across SH & DESH = 3.5
Calculated Economiser pressure drop = 1.5 kg/cm2 g
Control valve pressure drop assumed = 2 kg/cm2 g
Calculated Feed line pressure drop = 1 kg/cm2 g
Margin on BFP flow = 15 %
SAFETY VALVE RELIEVING CAPACITY CALCULATIONS
Maximum steam generation capacity of boiler = 35000 kg/h
Peak generation capacity = 1.1 x 35000 kg/h
= 38500 kg/h
MSSV relieving capacity = 0.4 x 38500 = 15400 kg/h
Saturated steam flow for deaerator = 400 kg/h
Drum SV 1 relieving capacity =( 0.3 x 38500) +( 0.5 x 400) = 11750 k
Drum SV 2 relieving capacity =( 0.3 x 38500) +( 0.5 x 400) = 11750 k
SAFETY VALVE SET PRESSURE & FEED PUMP DISCHARGE PRESSURE
Main steam pressure = 44 kg/cm2 g
MSSV set pressure = 44 / 0.95 kg/cm2 g
= 46.316 kg/cm2 g
MSSV set pressure = say, 46.5 kg/cm2 g
Calculated SH press. Drop = 3.5 kg / cm2
Drum pressure when MSSV floats = 46.5 + 3.5 = 50 kg/cm2 g
Margin = 1 kg/cm2 g
Selected pressure = 50 + 1 = 51 kg/cm2 g
Drum I SV set pressure = 51 / 0.95 kg/cm2 g
Drum I SV set pressure = 53.7 kg/cm2 g
Say, Drum I safety valve set pressure = 54 kg/cm2 g
Difference between drum I & II SV set pressures = 1 kg/cm2 g
Drum II SV set pressure = 54+1 = 55 kg/cm2 g
Economiser pressure drop = 1.5 kg/cm2g
Control valve pressure drop = 2 kg/cm2g
Feed line pressure drop = 1 kg/cm2g